Add getUniform impl methods.

This will let us remove some of the uniform data management code in
the GL front-end, and simplify the GL back-end. It will also enable
us to implement uniform data more efficiently in the D3D11 back-end,
and probably Vulkan back-end later.

This also implements a new impl method for the ProgramGL class to
flag optimized-out uniforms as no longer used, post-link. This is
important because otherwise the optimized uniforms get assigned
valid locations, and then the getUniform calls are expected to
succeed.

We also use a workaround for uniform value queries for the GL
back-end. It seems as though some drivers (seen on NVIDIA and AMD)
may not properly clamp to the maximum representable integer value
when querying out-of-range floating point values. Work around this by
always calling the driver with the proper type and then casting the
value in ANGLE.

BUG=angleproject:1390

Change-Id: I03dc2382e7af52455c356a2bf3971a4d1bd46ec6
Reviewed-on: https://chromium-review.googlesource.com/616785
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
diff --git a/src/tests/gl_tests/ComputeShaderTest.cpp b/src/tests/gl_tests/ComputeShaderTest.cpp
index f7326a2..7e677ee 100644
--- a/src/tests/gl_tests/ComputeShaderTest.cpp
+++ b/src/tests/gl_tests/ComputeShaderTest.cpp
@@ -77,11 +77,13 @@
 
     ANGLE_GL_COMPUTE_PROGRAM(program, csSource);
 
-    GLint uniformLoc = glGetUniformLocation(program.get(), "myUniformInt");
-    EXPECT_NE(-1, uniformLoc);
+    // It's not possible to validate uniforms are present since they are unreferenced.
+    // TODO(jmadill): Make uniforms referenced.
+    // GLint uniformLoc = glGetUniformLocation(program.get(), "myUniformInt");
+    // EXPECT_NE(-1, uniformLoc);
 
-    uniformLoc = glGetUniformLocation(program.get(), "myUniformSampler");
-    EXPECT_NE(-1, uniformLoc);
+    // uniformLoc = glGetUniformLocation(program.get(), "myUniformSampler");
+    // EXPECT_NE(-1, uniformLoc);
 
     EXPECT_GL_NO_ERROR();
 }